Method and device for determining configuration of connection between terminal and base station and performing handover in wireless communication system supporting dual connectivity

ABSTRACT

Provided are a method and apparatus for determining connection configuration between a user equipment (UE) and a base station (ENB) and performing handover in a wireless communication system supporting dual connectivity. The method of setting a connection configuration for a UE in a wireless communication system supporting dual connectivity may include: detecting occurrence of a handover event; selecting a macro ENB with the highest signal strength among neighboring macro ENBs and a small ENB with the highest signal strength among neighboring small ENBs; checking whether dynamic association is allowed in dual connectivity; and determining the connection configuration for the UE on the basis of the selected ENBs and checking result.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more particularly, to a method and apparatus for determining connectionconfiguration between a user equipment and a base station and performinghandover in a wireless communication system supporting dualconnectivity.

BACKGROUND ART

In general, mobile communication systems have been developed to providevoice services while guaranteeing user mobility. Such mobilecommunication systems have gradually expanded their coverage from voiceservices through data services up to high-speed data services. However,as current mobile communication systems suffer resource shortages andusers demand even higher-speed services, development of more advancedmobile communication systems is needed.

Meanwhile, dual connectivity enables a user equipment (UE) to connect totwo different base stations (ENBs) and to receive services therefrom atthe same time. For example, a dual connectivity enabled UE may connectto macro and small ENBs having different functions and receive servicestherefrom.

Standardization bodies for communication are currently active toinvestigate various technologies for dual connectivity. In particular,it is crucially necessary to develop a scheme for determining connectionconfiguration between UE and ENB and for handling handover.

DISCLOSURE OF INVENTION Technical Problem

The present invention has been made in view of the above problems.Accordingly, an aspect of the present invention is to provide a methodand apparatus for determining connection configuration between a userequipment (UE) and a base station (BS or ENB) and handling handover in awireless communication system supporting dual connectivity.

More specifically, an aspect of the present invention is to provide ascheme that enables a UE having discovered a new ENB to determinewhether to have single connectivity or dual connectivity, whether tomaintain single or dual connectivity to a particular ENB, the point intime for connection switching, and information needed for connectionswitching.

Solution to Problem

In accordance with an aspect of the present invention, there is provideda method of setting a connection configuration for a user equipment (UE)in a wireless communication system supporting dual connectivity. Themethod may include: detecting occurrence of a handover event; selectinga macro base station (macro ENB) with the highest signal strength amongneighboring macro ENBs and a small base station (small ENB) with thehighest signal strength among neighboring small ENBs; checking whetherdynamic association is allowed in dual connectivity; and determining theconnection configuration for the UE on the basis of the selected ENBsand checking result.

In accordance with another aspect of the present invention, there isprovided a method of setting a connection configuration for a basestation (ENB) in a wireless communication system supporting dualconnectivity. The method may include: receiving a measurement reportfrom a user equipment (UE) wherein the measurement report containsresults of measurement performed by the UE on a macro ENB and a smallENB; determining the connection configuration for the UE on the basis ofthe measurement report; and sending the determined connectionconfiguration to the UE.

In accordance with another aspect of the present invention, there isprovided a user equipment (UE) capable of setting a connectionconfiguration in a wireless communication system supporting dualconnectivity. The user equipment may include: a transceiver unit to sendand receive signals to and from a base station (ENB); and a control unitto control a process of selecting, upon detecting a handover event, amacro ENB with the highest signal strength and a small ENB with thehighest signal strength, checking whether dynamic association is allowedin dual connectivity, and determining the connection configuration forthe UE on the basis of the selected ENBs and checking result.

In accordance with another aspect of the present invention, there isprovided a base station (ENB) capable of setting a connectionconfiguration for a user equipment (UE) in a wireless communicationsystem supporting dual connectivity. The base station may include: atransceiver unit to send and receive a signal to and from the UE; and acontrol unit to control a process of receiving a measurement reportcontaining results of measurement performed by the UE on a macro ENB anda small ENB from the UE, determining the connection configuration forthe UE on the basis of the measurement report, and sending thedetermined connection configuration to the UE.

Advantageous Effects of Invention

In a feature of the present invention, it is possible to make best useof dual connectivity while minimizing switching between singleconnectivity mode and dual connectivity mode. Thereby, the userequipment may remain in dual connectivity mode for an extended time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates advantages of using dual connectivity in handoveraccording to an embodiment of the present invention.

FIG. 2 illustrates a situation where a UE selects a serving cell inconsideration of multiple ENBs at the same time according to anembodiment of the present invention.

FIG. 3 illustrates a situation where a dual connectivity enabled UEsearches for a new ENB in the vicinity according to an embodiment of thepresent invention.

FIG. 4 illustrates types of connection configurations selectable by theUE upon discovery of a macro ENB with higher signal strength/qualitythan the current macro ENB (cell) according to an embodiment of thepresent invention.

FIG. 5 illustrates types of connection configurations selectable by theUE upon discovery of a small ENB with higher signal strength/qualitythan the current small ENB according to an embodiment of the presentinvention.

FIG. 6 illustrates types of connection configurations selectable by theUE upon discovery of macro and small ENBs with higher signalstrength/quality than the current macro and small ENBs according to anembodiment of the present invention.

FIG. 7 is a flowchart of a procedure for the UE to determine aconnection configuration according to an embodiment of the presentinvention.

FIG. 8 is a flowchart of a procedure for the UE to determine aconnection configuration according to another embodiment of the presentinvention.

FIG. 9 is a flowchart of a procedure for the UE to determine aconnection configuration according to another embodiment of the presentinvention.

FIG. 10 is a flowchart of a procedure for the UE to perform measurementreporting according to an embodiment of the present invention.

FIG. 11 illustrates the point in time to send a measurement reportaccording to an embodiment of the present invention.

FIG. 12 illustrates transmission of cell pair information for macro andsmall ENBs according to an embodiment of the present invention.

FIG. 13 is a sequence diagram of an overall procedure for the UE todetermine a connection configuration and perform measurement reportingaccording to an embodiment of the present invention.

FIG. 14 illustrates the point in time for the UE to send a measurementreport according to an embodiment of the present invention.

FIG. 15 is a block diagram of a user equipment according to anembodiment of the present invention.

FIG. 16 is a block diagram of a base station according to an embodimentof the present invention.

MODE FOR THE INVENTION

Hereinafter, preferred embodiments of the present invention aredescribed in detail with reference to the accompanying drawings. Thesame or similar reference symbols are used throughout the drawings torefer to the same or like parts. Descriptions of well-known functionsand constructions may be omitted for clarity and conciseness.

Descriptions of well-known functions and structures incorporated hereinmay be omitted to avoid obscuring the subject matter of the presentinvention. Descriptions of components having substantially the sameconfigurations and functions may also be omitted.

In the drawings, some elements are exaggerated, omitted, or onlyoutlined in brief, and thus may be not drawn to scale. The presentinvention is not limited by the relative sizes of objects and intervalsbetween objects in the drawings.

The aspects, features and advantages of certain embodiments of thepresent invention will be more apparent from the following detaileddescription taken in conjunction with the accompanying drawings. Thedescription of the various embodiments is to be construed as exemplaryonly and does not describe every possible instance of the presentinvention. It should be apparent to those skilled in the art that thefollowing description of various embodiments of the present invention isprovided for illustration purpose only and not for the purpose oflimiting the present invention as defined by the appended claims andtheir equivalents. The same reference symbols are used throughout thedescription to refer to the same parts.

Meanwhile, it is known to those skilled in the art that blocks of aflowchart (or sequence diagram) and a combination of flowcharts may berepresented and executed by computer program instructions. Thesecomputer program instructions may be loaded on a processor of a generalpurpose computer, special purpose computer or programmable dataprocessing equipment. When the loaded program instructions are executedby the processor, they create a means for carrying out functionsdescribed in the flowchart. As the computer program instructions may bestored in a computer readable memory that is usable in a specializedcomputer or a programmable data processing equipment, it is alsopossible to create articles of manufacture that carry out functionsdescribed in the flowchart. As the computer program instructions may beloaded on a computer or a programmable data processing equipment, whenexecuted as processes, they may carry out steps of functions describedin the flowchart.

A block of a flowchart may correspond to a module, a segment or a codecontaining one or more executable instructions implementing one or morelogical functions, or to a part thereof. In some cases, functionsdescribed by blocks may be executed in an order different from thelisted order. For example, two blocks listed in sequence may be executedat the same time or executed in reverse order.

In the description, the word “unit”, “module” or the like may refer to asoftware component or hardware component such as an FPGA or ASIC capableof carrying out a function or an operation. However, “unit” or the likeis not limited to hardware or software. A unit or the like may beconfigured so as to reside in an addressable storage medium or to driveone or more processors. Units or the like may refer to softwarecomponents, object-oriented software components, class components, taskcomponents, processes, functions, attributes, procedures, subroutines,program code segments, drivers, firmware, microcode, circuits, data,databases, data structures, tables, arrays or variables. A functionprovided by a component and unit may be a combination of smallercomponents and units, and may be combined with others to compose largecomponents and units. Components and units may be configured to drive adevice or one or more processors in a secure multimedia card.

As described before, dual connectivity is one of various technologiesbeing under active investigation. Some techniques for coordinatedtransmission and reception have already been proposed. Coordinatedmulti-point transmission and reception (CoMP) is a representativeexample.

While ENBs having the same functions cooperate to support a UE in CoMP,macro and small ENBs having different functions cooperate to support aUE in dual connectivity.

This difference causes new problems to be solved.

Embodiments of the present invention to be described below propose ascheme for determining a connection configuration between UE and ENBsand a scheme for handling handover in a mobile communication systemsupporting dual connectivity.

More specifically, a description is given of a scheme that enables a UEhaving discovered a new ENB to determine whether to use singleconnectivity or dual connectivity, which ENB the UE should maintainsingle or dual connectivity to, the point in time for connectionswitching, and information needed for connection switching. In addition,a description is given of a scheme that enables a mobile communicationsystem scheduled to employ dual connectivity to effectively support UEmobility.

Hereinafter, the scheme of the present invention for determining aconnection configuration in dual connectivity and for handover isdescribed in the following order.

(a) Background: dual connectivity and handover

(b) Proposed scheme

<b1> Determining connection configuration in dual connectivity

<b2> Triggering handover in dual connectivity

<b3> Information provided for proposed scheme

(c) Summary

(a) Background: Dual Connectivity and Handover

As described before, dual connectivity enables a UE to simultaneouslyconnect to macro and small ENBs to receive services therefrom. Here, themacro ENB and small ENB have differences not only in cell coverage (orradius of service) but also in function. Specifically, for a UE havingdual connectivity to a macro ENB and small ENB at the same time, a RadioResource Control (RRC) message may be sent only to the macro ENB. Thesmall ENB is not allowed to directly send an RRC message to the UEhaving dual connectivity, but can play a part in RRC messagetransmission through information exchange with the macro ENB. In otherwords, the macro ENB is responsible for the RRC function of a UE havingdual connectivity.

For reference, RRC functions are described in Table 1.

TABLE 1 Broadcast of System Information related to the non-accessstratum (NAS) Broadcast of System Information related to the accessstratum (AS) Paging Establishment, maintenance and release of an RRCconnection between UE and E-UTRAN Security functions including keymanagement Establishment, configuration, maintenance and release ofpoint to point radio bearers Mobility functions QoS management functionsUE measurement reporting and control of reporting Direct NAS messagetransfer to/from NAS from/to UE

A UE having dual connectivity may have the following advantages inhandover. These advantages are described with reference to FIG. 1.

FIG. 1 illustrates advantages of using dual connectivity in handoveraccording to an embodiment of the present invention.

The advantage is described first with reference to part (a) of FIG. 1.

First, as the macro ENB 110 is responsible for RRC message transfer toand from the UE 100, radio link failure (RLF) occurring at the linkbetween the UE 100 and small ENB 120 does not cause trouble to RRCmessage transfer of the UE 100.

The advantage is described further with reference to part (b) of FIG. 1.

Second, when an RLF occurs at the link between the UE 100 and macro ENB110, RRC messages may be transferred to and from the UE 100 through therelayed link (UE 100—small ENB 120—macro ENB 110). That is, RLFoccurring at the link between the UE and macro ENB may delay RRC messagetransmission owing to the relayed link but does not cause a failure inRRC message transmission.

As described above, the UE 100 having dual connectivity may stably sendand receive RRC messages. Hence, it is possible to significantly reducethe possibility of handover failure due to the loss of handover-relatedRRC messages.

To make good use of dual connectivity, it is necessary for the UE toselect one or more serving cells in consideration of multiple ENBs atthe same time. This is described with reference to FIG. 2.

FIG. 2 illustrates a situation where the UE selects a serving cell inconsideration of multiple ENBs at the same time according to anembodiment of the present invention.

As shown in parts (a) and (b) of FIG. 2, it is assumed that the macroENB 210 and small ENB 220 belonging to cell pair 1 is interconnected viaa backhaul link and the macro ENB 230 and small ENB 240 belonging tocell pair 2 is interconnected. Each cell pair is assumed to enable theUE 200 to have dual connectivity. In other words, in one embodiment, acell pair may be formed of a macro ENB and small ENB that enable one UEto have dual connectivity.

It is also assumed that ENBs belonging to different cell pairs does notenable one UE to have dual connectivity owing to backhaul delay or thelike because the ENBs are not interconnected or are interconnected via alink with two or more hops.

Under the above assumptions, a description is given of the situationshown in part (a) of FIG. 2.

Part (a) of FIG. 2 depicts a situation where the UE 200 has discovered amacro ENB of cell pair 1 with the highest signal strength/quality and asmall ENB of cell pair 2 with the highest signal strength/quality.

If the UE 200 is of low mobility and is sending and receiving high-speeddata, it may prefer receiving a service from a small ENB to receiving aservice from a macro ENB. Hence, the UE 200 may attempt to connect tothe small ENB 240 belonging to cell pair 2.

In this case, to obtain gains due to dual connectivity, it may bepreferable for the UE 200 to connect to the macro ENB 230 of cell pair 2rather than the macro ENB 210 of cell pair 1 with the highest signalstrength/quality.

Next, a description is given of the situation shown in part (b) of FIG.2.

Part (b) of FIG. 2 depicts a situation where the UE 200 has discovered asmall ENB 220 of cell pair 1 with the highest signal strength/qualityand a macro ENB 230 of cell pair 2 with the highest signalstrength/quality.

If the UE 200 is of high mobility and is sending and receiving a smallamount of data, it may prefer receiving a service from a macro ENB toreceiving a service from a small ENB. Hence, the UE 200 may attempt toconnect to the macro ENB 230 belonging to cell pair 2.

In this case, to obtain gains due to dual connectivity, it may bepreferable for the UE 200 to connect to the small ENB 240 of cell pair 2rather than the small ENB 220 of cell pair 1 with the highest signalstrength/quality.

This phenomenon is caused by the difference between roles of the macroENB and small ENB. That is, as the macro ENB is responsible for the RRCfunction of a UE having dual connectivity, to obtain RRC diversity gainsdue to dual connectivity, it sometimes happens that the UE is notconnected to the ENB with the highest signal strength/quality.

In the existing system supporting single connectivity only, handover isgenerally carried out so that the UE is connected to the ENB with thehighest signal strength/quality. However, in a system supporting dualconnectivity as described above, the signal strength/quality may alonebe insufficient as the criterion for handover.

To address such a problem caused by dual connectivity, in the presentinvention, a description is given of a scheme for determining theconnection configuration and performing handover in the case of dualconnectivity.

(b) Proposed Scheme

<b1> Determining Connection Configuration in Dual Connectivity

FIG. 3 illustrates a situation where a dual connectivity enabled UEsearches for a new ENB in the vicinity according to an embodiment of thepresent invention.

The UE may discover a new ENB in three situations depicted respectivelyin parts (a), (b) and (c) of FIG. 3.

Part (a) of FIG. 3 illustrates a situation where the UE discovers amacro ENB with higher signal strength/quality than the current macro ENB(cell).

Part (b) of FIG. 3 illustrates a situation where the UE discovers asmall ENB with higher signal strength/quality than the current smallENB.

Part (c) of FIG. 3 illustrates a situation where the UE discovers amacro ENB and small ENB with higher signal strength/quality than thecurrent macro ENB and small ENB.

Next, a description is given of connection configurations available ineach of the situations depicted in parts (a), (b) and (c) of FIG. 3.

First, a description is given of the situation where the UE discovers amacro ENB with higher signal strength/quality than the current macro ENB(macro cell) with reference to FIG. 4.

FIG. 4 illustrates types of connection configurations selectable by theUE upon discovery of a macro ENB with higher signal strength/qualitythan the current macro ENB (cell) according to an embodiment of thepresent invention.

The UE 400 may discover a macro ENB 430 with higher signalstrength/quality than the current macro ENB 410 as shown in part (a) ofFIG. 4. In this case, the UE 400 may make a connection according to oneof the following ways.

As shown in part (b) of FIG. 4, the UE 400 may determine to have singleconnectivity. Here, the UE 400 may make a single connection to the newlydiscovered macro ENB 430.

Part (b) of FIG. 4 may correspond to a case where it is not necessary tomaintain dual connections because the newly discovered macro ENB 430 hasan excellent signal strength/quality, or to a case where the connectionto a small ENB is not needed owing to high UE mobility.

As shown in part (c) of FIG. 4, the UE 400 may determine to maintain thecurrent connection configuration in consideration of dual connectivityand static association. That is, the UE 400 may maintain dualconnectivity to the macro ENB 410 and small ENB 420 belonging to cellpair 1.

Part (c) of FIG. 4 may correspond to a case where the good signalquality of the newly discovered macro ENB 430 is not very useful becausethe UE 400 receives a service mainly from the current small ENB 420.

As shown in part (d) of FIG. 4, the UE 400 may determine to set a newconnection configuration in consideration of dual connectivity andstatic association. To utilize the newly discovered macro ENB 430, theUE 400 may have new dual connectivity by connecting to the macro ENB 430and small ENB 440 belonging to cell pair 2.

Part (d) of FIG. 4 may correspond to a case where the UE 400 prefers toconnect to the newly discovered macro ENB 430 so as to obtain RRCdiversity gains due to dual connectivity although the small ENB 420 hasa higher signal strength/quality than the other small ENBs. Here, it isnecessary to make dual connections to the macro ENB 430 and small ENB440 belonging to the same cell pair (i.e. cell pair 2).

As shown in part (e) of FIG. 4, the UE 400 may determine to set a newconnection configuration in consideration of dual connectivity anddynamic association. When dynamic association is allowed, the UE 400 mayhave new dual connectivity by conducting connection switching from themacro ENB 410 to the macro ENB 430 of cell pair 2 and maintaining theconnection to the small ENB 420 of cell pair 1.

This dynamic association may be most desirable in terms of signalstrength/quality between UE and macro/small ENB, but it requireslow-delay communication between ENBs belonging to different cell pairsthrough the wired/wireless backhaul.

FIG. 5 illustrates types of connection configurations selectable by theUE upon discovery of a small ENB with higher signal strength/qualitythan the current small ENB according to an embodiment of the presentinvention.

The UE 500 may discover a small ENB 530 with higher signalstrength/quality than the current small ENB 520 as shown in part (a) ofFIG. 5. In this case, the UE 500 may make a connection according to oneof the following ways.

As shown in part (b) of FIG. 5, the UE 500 may make a single connectionto the newly discovered small ENB 540. This may correspond to a casewhere it is not necessary to maintain dual connections because the newlydiscovered small ENB 540 has an excellent signal strength/quality, or toa case where the connection to a macro ENB is not needed owing to low UEmobility or offloading of the macro ENB.

As shown in part (c) of FIG. 5, the UE 500 may maintain the currentconnection configuration in consideration of dual connectivity andstatic association. That is, the UE 500 may maintain dual connectivityto the macro ENB 510 and small ENB 520 belonging to current cell pair 1.This may correspond to a case where the good signal quality of the newlydiscovered small ENB 540 is not very useful because the UE 500 receivesa service mainly from the current macro ENB 510.

As shown in part (d) of FIG. 5, the UE 500 may set a new connectionconfiguration in consideration of dual connectivity and staticassociation. That is, the UE 500 may have new dual connectivity byconnecting to the macro ENB 530 and small ENB 540 belonging to cell pair2.

This may correspond to a case where the UE 500 prefers to connect to thenewly discovered small ENB 540 so as to obtain RRC diversity gains dueto dual connectivity although the macro ENB 510 has a higher signalstrength/quality than the macro ENB 530. Here, it is necessary to makedual connections to the macro ENB and small ENB belonging to the samecell pair (i.e. cell pair 2).

As shown in part (e) of FIG. 5, the UE 400 may set a new connectionconfiguration in consideration of dual connectivity and dynamicassociation. That is, the UE 500 may have new dual connectivity byconducting connection switching to the small ENB 540 of cell pair 2 andmaintaining the connection to the macro ENB 510 of cell pair 1.

This dynamic association may be most desirable in terms of signalstrength/quality between UE and macro/small ENB, but it requireslow-delay communication between ENBs belonging to different cell pairsthrough the wired/wireless backhaul.

FIG. 6 illustrates types of connection configurations selectable by theUE upon discovery of macro and small ENBs with higher signalstrength/quality than the current macro and small ENBs according to anembodiment of the present invention.

As shown in part (a) of FIG. 6, the UE 600 may discover a macro ENB 630and small ENB 640 with higher signal strength/quality than the currentmacro ENB 610 and small ENB 620.

In this case, as shown in part (b) of FIG. 6, the UE 500 may set a newconnection configuration to utilize the newly discovered macro and smallENBs in consideration of dual connectivity and static association. Thatis, the UE 500 may have new dual connectivity by connecting to the macroENB 630 and small ENB 640 belonging to cell pair 2.

Hereinabove, a description is given of types of connectionconfigurations available when a dual connectivity enabled UE discovers anew macro ENB or new small ENB with reference to FIGS. 4 to 6.

In the existing system supporting single connectivity only, the UE hasonly to select one ENB with the highest signal strength/quality and doesnot have to consider a suitable connection configuration. However, in asystem supporting dual connectivity according to the present invention,as various connection configurations are possible, the UE has to selectone of the connection configurations.

To address the above issue, a description is given of a scheme of thepresent invention for determining the connection configuration withreference to a flowchart of FIG. 7.

FIG. 7 is a flowchart of a procedure for the UE to determine aconnection configuration according to an embodiment of the presentinvention.

At step S710, the UE selects a macro ENB with the highest signalstrength/quality among neighboring macro ENBs. At step S720, the UEselects a small ENB with the highest signal strength/quality amongneighboring small ENBs.

At step S730, the UE determines whether dynamic association is allowed.

Dynamic association may refer to a mechanism that permits the UE to havedual connectivity to a macro ENB and small ENB belonging to differentcell pairs. For example, when dynamic association is allowed, the UE mayconnect to a macro ENB of a first cell pair and a small ENB of a secondcell pair. Likewise, when dynamic association is allowed, the UE mayconnect to a small ENB of the first cell pair and a macro ENB of thesecond cell pair.

Whether dynamic association is allowed can be identified in variousways. For example, the network may send the UE a separate signalindicating whether dynamic association is allowed. The network maytransmit system information containing information indicating whetherdynamic association is allowed.

Alternatively, the UE may send the network a separate query aboutwhether dynamic association is allowed. In response to the query, thenetwork may send the UE a message indicating whether dynamic associationis allowed.

If dynamic association is allowed, at step S740, the UE makes dualconnections to the macro ENB selected at step S710 and the small ENBselected at step S720.

If dynamic association is not allowed, at step S750, the UE selects theENB with the highest signal strength/quality. Here, the UE may selectone of the macro ENB selected at step S710 and the small ENB selected atstep S720 having a higher signal strength/quality.

At step S760, the UE identifies the ENB paired with the selected ENB andchecks whether the ENB paired with the selected ENB has a signalstrength/quality higher than or equal to a preset threshold.

If the paired ENB has a signal strength/quality higher than or equal tothe preset threshold, at step S780, the UE makes dual connections to theENB with the highest signal strength/quality (best ENB) and the ENBpaired therewith.

If the paired ENB has a signal strength/quality lower than or equal tothe preset threshold, at step S770, the UE makes a single connection tothe ENB with the highest signal strength/quality (best ENB).

The UE may select one connection configuration among multiple possibleconnection configurations according to the above procedure.

In another embodiment of the present invention, the following issue maybe further applied in consideration of characteristics of the macro ENBand small ENB.

Specifically, it is desirable for a high-mobility UE to connect to amacro ENB having a wide coverage. Hence, in this case, the UE may beforced to select a macro ENB as the best ENB (at step S750).

This embodiment is described in FIG. 8.

FIG. 8 is a flowchart of a procedure for the UE to determine aconnection configuration according to another embodiment of the presentinvention. More specifically, in FIG. 8, it is assumed that the UE is ofhigh mobility.

At step S810, the UE selects a macro ENB with the highest signalstrength/quality among neighboring macro ENBs. At step S820, the UEselects a small ENB with the highest signal strength/quality amongneighboring small ENBs.

At step S830, the UE determines whether dynamic association is allowed.

If dynamic association is allowed, at step S840, the UE makes dualconnections to the macro ENB selected at step S810 and the small ENBselected at step S820.

If dynamic association is not allowed, at step S850, the UE selects themacro ENB selected at step S810. This is because the UE is of highmobility.

At step S860, the UE identifies the ENB paired with the selected ENB andchecks whether the ENB paired with the selected ENB has a signalstrength/quality higher than or equal to a preset threshold.

If the paired ENB has a signal strength/quality higher than or equal tothe preset threshold, at step S880, the UE makes dual connections to theselected macro ENB and the ENB paired therewith.

If the paired ENB has a signal strength/quality lower than or equal tothe preset threshold, at step S870, the UE makes a single connection tothe selected macro ENB (macro ENB with the highest signalstrength/quality).

In addition to UE mobility, the connection configuration may bedetermined in consideration of the data rate or offloading. When a UE isof low mobility and requires a high data rate, it is desirable for theUE to connect to a small ENB for offloading of the macro ENB.

Hence, in this case, the UE may be forced to select a small ENB as thebest ENB (at step S750 in FIG. 7).

This embodiment is described in FIG. 9.

FIG. 9 is a flowchart of a procedure for the UE to determine aconnection configuration according to another embodiment of the presentinvention. More specifically, in FIG. 9, it is assumed that the UE is oflow mobility.

At step S910, the UE selects a macro ENB with the highest signalstrength/quality among neighboring macro ENBs. At step S920, the UEselects a small ENB with the highest signal strength/quality amongneighboring small ENBs.

At step S930, the UE determines whether dynamic association is allowed.

If dynamic association is allowed, at step S940, the UE makes dualconnections to the macro ENB selected at step S910 and the small ENBselected at step S920.

If dynamic association is not allowed, at step S950, the UE selects thesmall ENB selected at step S920. This is because the UE is of lowmobility.

At step S960, the UE identifies the ENB paired with the selected ENB andchecks whether the ENB paired with the selected ENB has a signalstrength/quality higher than or equal to a preset threshold.

If the paired ENB has a signal strength/quality higher than or equal tothe preset threshold, at step S980, the UE makes dual connections to theselected small ENB and the ENB paired therewith.

If the paired ENB has a signal strength/quality lower than or equal tothe preset threshold, at step S970, the UE makes a single connection tothe selected small ENB (small ENB with the highest signalstrength/quality).

As described above, it is possible to force the UE to prefer a specificconnection configuration by placing a limitation on the selectable bestENB according to mobility or offloading.

<b2> Triggering Handover in Dual Connectivity

Hereinabove, a description is given of a scheme for determining theconnection configuration in dual connectivity.

After determining the connection configuration, when there is a need forchanging the connection configuration, this is to be notified to the ENBin the form of a measurement report. In the case of single connectivity,when the UE discovers an ENB having a higher signal strength/qualitythan the serving ENB and this state is maintained for a preset time(time-to-trigger, TTT), the UE sends a measurement report to the servingENB to thereby trigger handover.

However, in the case of dual connectivity, the scheme for measurementreporting may be varied according to the connection configurationselected by the UE as described below.

First, when the UE maintains the current connection configuration as aresult of determining the connection configuration described above,there is no need to send a measurement report to the serving ENB. Inthis case, the UE does not send a measurement report to the serving ENB.

Second, when the UE determines to have single connectivity or to havedual connectivity based on dynamic association, the UE changes one ofthe links to the macro ENB and the small ENB. Hence, the UE sends theserving ENB a measurement report for the link to be changed.

That is, when the UE determines to make a connection to a new macro ENB,the UE sends the serving ENB results of measurement on frequency bandsof the macro ENB. When the UE determines to make a connection to a newsmall ENB, the UE sends the serving ENB results of measurement onfrequency bands of the small ENB.

Third, when the UE determines to have dual connectivity based on staticassociation, the UE changes both of the links to the macro ENB and thesmall ENB. Hence, the UE sends the serving ENB results of measurement onthe macro ENB and the small ENB.

The above scheme for measurement reporting based on the connectionconfiguration is described in FIG. 10.

FIG. 10 is a flowchart of a procedure for the UE to perform measurementreporting according to an embodiment of the present invention.

Referring to FIG. 10, at step S1000, the UE identifies the connectionconfiguration. Here, the connection configuration may indicate one ofsingle connectivity, dual connectivity based on static association, anddual connectivity based on dynamic association.

At step S1010, the UE checks whether the current connectionconfiguration is maintained. If the current connection configuration ismaintained, at step S1020, the UE skips measurement reporting. That is,the UE does not send a measurement report to the serving ENB.

If the current connection configuration is not maintained, at stepS1030, the UE checks whether the connection configuration indicatessingle connectivity. If the connection configuration indicates singleconnectivity, at step S1040, the UE sends the serving ENB a measurementreport for a single layer. Here, the “layer” may denote a frequency or afrequency band.

Specifically, when the UE determines to have single connectivity, the UEchanges one of the links to the macro ENB and the small ENB. Hence, theUE sends the serving ENB a measurement report only for the link to bechanged.

If the connection configuration does not indicate single connectivity,at step S1050, the UE checks whether the connection configurationindicates dual connectivity based on dynamic association. If theconnection configuration indicates dual connectivity based on dynamicassociation, at step S1060, the UE sends the serving ENB a measurementreport for a single layer. Specifically, when the UE determines to havedual connectivity based on dynamic association, the UE changes one ofthe links to the macro ENB and the small ENB. Hence, the UE sends theserving ENB a measurement report only for the link to be changed.

If the connection configuration indicates dual connectivity based onstatic association, at step S1070, the UE sends the serving ENB resultsof measurement on the macro ENB and the small ENB.

Meanwhile, it is necessary to determine the point in time at which theUE sends a measurement report to the serving ENB. This is described withreference to FIG. 11.

FIG. 11 illustrates the point in time to send a measurement reportaccording to an embodiment of the present invention.

In the case of single connectivity, after measurement on the macro ENBlayer (macro ENB frequency band), when the UE discovers a macro ENBhaving a higher signal strength/quality than the serving macro ENB andthis state is maintained for the TTT (or first timer), the UE sends ameasurement report for the macro ENB layer at the time of TTTexpiration.

Likewise, after measurement on the small ENB layer (small ENB frequencyband), when the UE discovers a small ENB having a higher signalstrength/quality than the serving small ENB and this state is maintainedfor the TTT (or second timer), the UE sends a measurement report for thesmall ENB layer at the time of TTT expiration.

In the case of dual connectivity, at the time of TTT expiration for themacro ENB layer, according to the determined connection configuration,the UE may not send a measurement report (i), may send a measurementreport for the macro ENB layer (ii), or may send a measurement reportfor the macro ENB layer and the small ENB layer (iii).

Likewise, at the time of TTT expiration for the small ENB layer,according to the determined connection configuration, the UE may notsend a measurement report (i), may send a measurement report for thesmall ENB layer (ii), or may send a measurement report for the macro ENBlayer and the small ENB layer (iii).

<b3> Information Provided for Proposed Scheme

In the present invention, whether a UE can have dual connectivity to amacro ENB and small ENB is determined on the basis of the cell pair.That is, when a macro ENB and a small ENB belonging to the same cellpair are interconnected through the backhaul and can exchange controlinformation within a given delay bound, the UE may have dualconnectivity to the macro ENB and the small ENB.

Hence, each of the macro ENB and the small ENB should notify the UE ofthe ENB paired therewith.

To this end, a macro ENB may broadcast the BS ID and physical cell ID(e.g. cell specific sequence, ID or information used for thecell-specific reference signal) of a small ENB pairable with the macroENB. In one embodiment, a macro ENB may broadcast information on a smallENB by use of the neighbor cell list.

Similarly, a small ENB may broadcast the BS ID and physical cell ID of amacro ENB pairable with the small ENB.

Transmission of cell pair information for a macro ENB and small ENB isdescribed with reference to FIG. 12.

FIG. 12 illustrates transmission of cell pair information for macro andsmall ENBs according to an embodiment of the present invention.

In FIG. 12, there are macro ENBs M1 to M3 and small ENBs S1 to S4 andENBs that can form a cell pair are linked by lines. In this case, eachENB may transmit their cell pair information as follows.

-   -   Macro ENB 1 (M1): BS IDs and physical cell IDs of small ENBs S1,        S2, S3    -   Macro ENB 2 (M2): BS IDs and physical cell IDs of small ENBs S1,        S2, S4    -   Macro ENB 3 (M3): BS IDs and physical cell IDs of small ENBs S1,        S3, S4    -   Small ENB 1 (S1): BS IDs and physical cell IDs of macro ENBs M1,        M2, M3    -   Small ENB 2 (S2): BS IDs and physical cell IDs of macro ENBs M1,        M2    -   Small ENB 3 (S3): BS IDs and physical cell IDs of macro ENBs M1,        M3    -   Small ENB 4 (S4): BS IDs and physical cell IDs of macro ENBs M2,        M3

Upon receiving cell pair information, the UE may identify a small ENBpaired with a given macro ENB and a macro ENB paired with a given smallENB on the basis of the received cell pair information.

(c) Summary

FIG. 13 is a sequence diagram of an overall procedure for the UE todetermine a connection configuration and perform measurement reportingaccording to an embodiment of the present invention.

As described before, the present invention includes three constituents.The first is a scheme for determining the connection configuration indual connectivity, the second is a scheme for triggering handover indual connectivity through measurement reporting, and the third is ascheme for providing cell pair information for dual connectivity.

The overall procedure is described with reference to FIG. 13.

At step S1310, each macro ENB and each small ENB transmit their cellpair information (or cell association information). Each ENB may sendtheir cell pair information to the UE 1310 through broadcasting (e.g. assystem information), or higher or physical layer messaging. Cell pairinformation of each ENB may include information on heterogeneous ENBspairable with the ENB.

At step S1320, the serving macro ENB 1320 sends measurement controlinformation for measurement control to the UE 1310.

At step S1330, the UE 1310 performs measurement according to themeasurement control information.

At step S1340, the UE 1310 detects occurrence of a handover event. Afterexpiration of a preset time (e.g. TTT) from detection of the handoverevent, at step S1350, the UE 1310 determines the connectionconfiguration. As described before, the UE may determine the connectionconfiguration in consideration of at least one of signal quality,mobility (movement speed), and offloading.

Upon arrival of the measurement report time, at step S1360, the UE 1310sends a measurement report to the serving macro ENB 1320 according tothe determined connection configuration. As described before, accordingto the determined connection configuration, the UE 1310 may skipmeasurement reporting, send a measurement report for the single layer,or send a measurement report for the two layers.

In the above description, the UE determines the connectionconfiguration. However, it does not necessarily mean that the UE has todetermine the connection configuration. For example, the UE simply sendsa measurement result to the serving ENB, and the ENB may determine theconnection configuration in dual connectivity.

Next, a description is given of another embodiment in which the UE sendsa measurement result to the serving ENB and the ENB determines theconnection configuration in dual connectivity.

For a macro ENB, the condition to be satisfied for TTTmacro is given byEquation 1.

RSRPtarget,macro>RSRPserving,macro+Δ(both RSRP and RSRQapplicable)  [Equation 1]

In the case of existing single connectivity, when TTTmacro expires aftercontinuous satisfaction of the condition, the UE sends a measurementreport for the macro ENB frequency band to the serving ENB and theserving ENB initiates a procedure to handover the UE to the target macroENB.

In the case of dual connectivity, as described before, the RRC functionis provided by not the small ENB but the macro ENB, the handoverprocedure may differ from that for single connectivity.

That is, upon expiration of TTTmacro after continuous satisfaction ofthe condition, it may be necessary to handle not only handover for themacro ENB but also handover for the small ENB.

Hence, upon expiration of TTTmacro, it is necessary for the UE to sendthe serving ENB both a measurement report for the macro ENB frequencyband and a measurement report for the small ENB frequency band, so thatthe serving ENB have information sufficient for handling UE handover.

In one embodiment, upon expiration of TTTmacro, the UE may send theserving ENB the measurement results for all small ENBs having beenmeasured (option 1), or send the serving ENB the measurement results forthose small ENBs satisfying a preset condition (option 2). Then, theserving ENB may determine the connection configuration for the UE on thebasis of the received measurement results.

Here, the preset condition for a small ENB is satisfied whenRSRPsmall>threshold is maintained for the minimum TTT. In this case, theminimum TTT may be set separately from TTTmacro or TTTsmall.

This is described below with reference to FIG. 14.

-   -   Option 1: RSRPtarget,macro>RSRPserving,macro+Δ

After continuous satisfaction of the above condition during TTTmacro,the UE sends the serving ENB a measurement report including measurementresults for macro and small ENBs having been measured.

Then, the serving ENB determines the connection configuration for the UEand the UE may initiate handover according to the determination.

-   -   Option 2: RSRPtarget,macro>RSRPserving,macro+Δ

After continuous satisfaction of the above condition during TTTmacro,the UE sends the serving ENB a measurement report including measurementresults for macro ENBs having been measured and small ENBs satisfyingthe condition “RSRPsmall>threshold” for TTTminimum.

Then, the serving ENB determines the connection configuration for the UEand the UE may initiate handover according to the determination.

Next, for a small ENB, the condition to be satisfied for TTTsmall isgiven by Equation 2.

RSRPtarget,small>RSRPserving,small+Δ  [Equation 2]

The above statement may also be applied to the small ENB case. This canbe summarized as follows.

-   -   Option 1: RSRPtarget,small>RSRPserving,small+Δ

After continuous satisfaction of the above condition during TTTsmall,the UE sends the serving ENB a measurement report including measurementresults for macro and small ENBs having been measured.

Then, the serving ENB determines the connection configuration for the UEand the UE may initiate handover according to the determination.

-   -   Option 2: RSRPtarget,small>RSRPserving,small+Δ

After continuous satisfaction of the above condition during TTTsmall,the UE sends the serving ENB a measurement report including measurementresults for small ENBs having been measured and macro ENBs satisfyingthe condition “RSRPmacro>threshold” for TTTminimum.

Then, the serving ENB determines the connection configuration for the UEand the UE may initiate handover according to the determination.

FIG. 15 is a block diagram of a user equipment according to anembodiment of the present invention. As shown in FIG. 15, the UE mayinclude a transceiver unit 1510, a storage unit 1520, and a control unit1530.

The transceiver unit 1510 performs wireless data transmission andreception operations for the UE. The transceiver unit 1510 may include aradio frequency (RF) transmitter for upconverting the frequency of asignal to be transmitted and amplifying the signal, and an RF receiverfor low-noise amplifying a received signal and downconverting thefrequency of the received signal. The transceiver unit 1510 may forwarddata received through a wireless channel to the control unit 1530, andtransmit data output from the control unit 1530 through the wirelesschannel. In particular, the transceiver unit 1510 may receive cell pairinformation from a macro ENB or small ENB. The transceiver unit 1510 maysend the serving ENB a measurement report containing measurement resultsfor the serving and neighbor cells.

The storage unit 1520 may store programs and data needed for operationof the UE. The storage unit 1520 may be divided into a program regionand a data region.

The control unit 1530 may control signal flows between components of theUE so that the UE can operate according to embodiments of the presentinvention. Specifically, the control unit 1530 may control a series ofoperations to determine the connection configuration according to atleast one of received signal strengths of ENBs, mobility of the UE, andnecessity of offloading. To this end, the control unit 1530 may includea received signal strength measurer 1531, a connection configurationcontroller 1532, and a measurement reporting controller 1533.

The received signal strength measurer 1531 may measure signal strengthsor signal qualities of signals received from multiple ENBs includingserving ENBs and neighbor ENBs. Here, the ENB may be a macro ENB orsmall ENB.

The connection configuration controller 1532 may select a macro ENB withthe highest signal strength and a small ENB with the highest signalstrength. The connection configuration controller 1532 may check whetherdynamic association is allowed for dual connectivity, and determine theconnection configuration for the UE on the basis of the selected ENBsand checking result.

For example, when dynamic association is allowed, the connectionconfiguration controller 1532 may control the UE to have dualconnectivity to the macro ENB and small ENB with the highest signalstrength.

When dynamic association is not allowed, the connection configurationcontroller 1532 may select the ENB with the highest signal strength andchecks whether the ENB paired with the selected ENB has a signalstrength higher than or equal to a preset threshold. When the ENB pairedwith the selected ENB has a signal strength higher than or equal to thepreset threshold, the connection configuration controller 1532 maycontrol the UE to have dual connectivity to the ENB having the highestsignal strength and the ENB paired therewith.

When the ENB paired with the selected ENB has a signal strength lowerthan the preset threshold, the connection configuration controller 1532may control the UE to have single connectivity to the ENB having thehighest signal strength.

In one embodiment, the connection configuration controller 1532 maydetermine the connection configuration according to at least one ofreceived signal strength, UE mobility, and necessity of offloading. Forexample, when the UE has high mobility (i.e. moves at a speed higherthan a threshold), the connection configuration controller 1532 maycontrol the UE to make a connection to the macro ENB first. When trafficoffloading is necessary, the connection configuration controller 1532may control the UE to make a connection to the small ENB first.

The measurement reporting controller 1533 may control a series ofoperations to send a measurement report to the serving ENB on the basisof the connection configuration determined by the connectionconfiguration controller 1532. For example, when the current connectionconfiguration is maintained, the measurement reporting controller 1533may skip sending a measurement report. When the connection configurationindicates single connectivity or dual connectivity based on dynamicassociation, the measurement reporting controller 1533 may control theUE to send a measurement report containing measurement information of asingle frequency band. When the connection configuration indicates dualconnectivity based on static association, the measurement reportingcontroller 1533 may control the UE to send a measurement reportcontaining measurement information of multiple frequency bands.

In addition, when the timer for the macro ENB or small ENB expires, themeasurement reporting controller 1533 may control the UE to send ameasurement report containing measurement information of at least onefrequency band associated with the macro ENB or small ENB.

In FIG. 15, the control unit 1530 is depicted as having separateinternal blocks with different functions. However, the present inventionis not limited thereto or thereby. For example, the function of theconnection configuration controller 1532 may be performed by the controlunit 1530 itself.

FIG. 16 is a block diagram of a base station according to an embodimentof the present invention. As shown in FIG. 16, the ENB may include atransceiver unit 1610, a storage unit 1620, and a control unit 1630.

The transceiver unit 1610 performs wireless data transmission andreception operations for the ENB. The transceiver unit 1610 may includean RF transmitter for upconverting the frequency of a signal to betransmitted and amplifying the signal, and an RF receiver for low-noiseamplifying a received signal and downconverting the frequency of thereceived signal. The transceiver unit 1610 may forward data receivedthrough a wired or wireless channel to the control unit 1630, andtransmit data output from the control unit 1630 through a wirelesschannel. In particular, the transceiver unit 1610 may send a UE cellpair information including information on heterogeneous ENBs pairablewith the ENB.

The storage unit 1620 may store programs and data needed for operationof the ENB. The storage unit 1620 may be divided into a program regionand a data region.

The control unit 1630 may control signal flows between components of theENB so that the ENB can operate according to embodiments of the presentinvention. Specifically, the control unit 1630 may control a series ofoperations to send cell pair information to the UE. The control unit1630 may also control a series of operations to determine the connectionconfiguration for the UE on the basis of measurement results for macroENBs or small ENBs received from the UE. To this end, the control unit1630 may include a connection configuration controller 1631.

The connection configuration controller 1631 may determine theconnection configuration for the UE on the basis of measurement resultsfor macro ENBs and small ENBs measured by and received from the UE. Themeasurement results may be received from the UE when the timer for themacro ENB or small ENB expires. In determining the connectionconfiguration for the UE, the connection configuration controller 1631of the ENB corresponds to the connection configuration controller 1532of the UE, and a detailed description thereof is omitted.

The connection configuration controller 1631 may control the ENB to sendinformation on the determined connection configuration to the UE. Then,the UE may perform handover according to the information on theconnection configuration.

According to various embodiments of the present invention, it ispossible to make best use of dual connectivity while minimizingswitching between single connectivity mode and dual connectivity mode.Consequently, the user equipment may remain in dual connectivity modefor an extended time.

Hereinabove, various embodiments of the present invention have beenshown and described for the purpose of illustration without limiting thesubject matter of the present invention. It should be understood bythose skilled in the art that many variations and modifications of themethod and apparatus described herein will still fall within the spiritand scope of the present invention as defined in the appended claims andtheir equivalents.

1. A method of setting a connection configuration for a user equipment(UE) in a wireless communication system supporting dual connectivity,the method comprising: detecting occurrence of a handover event;selecting a macro base station (macro ENB) with the highest signalstrength among neighboring macro ENBs and a small base station (smallENB) with the highest signal strength among neighboring small ENBs;checking whether dynamic association is allowed in dual connectivity;and determining the connection configuration for the UE on the basis ofthe selected ENBs and checking result.
 2. The method of claim 1, whereindetermining the connection configuration further comprises causing, whendynamic association is allowed, the UE to have dual connectivity to themacro ENB with the highest signal strength and the small ENB with thehighest signal strength.
 3. The method of claim 1, wherein determiningthe connection configuration further comprises, when dynamic associationis not allowed: selecting an ENB with the highest signal strength;checking whether an ENB paired with the selected ENB has a signalstrength higher than or equal to a preset threshold; causing, when thepaired ENB has a signal strength higher than or equal to the threshold,the UE to have dual connectivity to the ENB with the highest signalstrength and the ENB paired therewith; and causing, when the paired ENBhas a signal strength lower than the threshold, the UE to have singleconnectivity to the ENB with the highest signal strength.
 4. The methodof claim 1, wherein determining the connection configuration comprisesdetermining the connection configuration for the UE on the basis ofmobility of the UE and necessity of traffic offloading, whereindetermining the connection configuration comprises causing the UE tomake a connection to a macro ENB first when the UE is of high mobility,and wherein determining the connection configuration comprises causingthe UE to make a connection to a small ENB first when traffic offloadingis necessary.
 5. The method of claim 1, further comprising receivingcell pair information from a macro ENB or small ENB, and wherein thecell pair information from a macro ENB comprises information on smallENBs pairable with the macro ENB, and the cell pair information from asmall ENB comprises information on macro ENBs pairable with the smallENB.
 6. The method of claim 1, further comprising sending a measurementreport to the serving ENB on the basis of the determined connectionconfiguration, wherein sending a measurement report is skipped when thecurrent connection configuration is maintained, wherein sending ameasurement report comprises transmitting a measurement reportcontaining measurement information for a single frequency band when theconnection configuration indicates single connectivity or dualconnectivity based on dynamic association, wherein sending a measurementreport comprises transmitting a measurement report containingmeasurement information for multiple frequency bands when the connectionconfiguration indicates dual connectivity based on static association,and wherein sending a measurement report comprises transmitting ameasurement report containing measurement information for at least onefrequency band associated with a macro ENB or small ENB at the time whenthe timer for the macro ENB or small ENB expires.
 7. A method of settinga connection configuration for a base station (ENB) in a wirelesscommunication system supporting dual connectivity, the methodcomprising: receiving a measurement report from a user equipment (UE)wherein the measurement report contains results of measurement performedby the UE on a macro ENB and a small ENB; determining the connectionconfiguration for the UE on the basis of the measurement report; andsending the determined connection configuration to the UE.
 8. The methodof claim 7, wherein the results of measurement on a small ENB compriseeither results of measurement on all small ENBs having been measured orresults of measurement on small ENBs satisfying a preset condition, andwherein receiving a measurement report comprising receiving themeasurement report at the time when the timer for the macro ENB or smallENB expires.
 9. A user equipment (UE) capable of setting a connectionconfiguration in a wireless communication system supporting dualconnectivity, comprising: a transceiver unit to send and receive signalsto and from a base station (ENB); and a control unit to control aprocess of selecting, upon detecting a handover event, a macro ENB withthe highest signal strength and a small ENB with the highest signalstrength, checking whether dynamic association is allowed in dualconnectivity, and determining the connection configuration for the UE onthe basis of the selected ENBs and checking result.
 10. The userequipment of claim 9, wherein, when dynamic association is allowed, thecontrol unit controls a process of having dual connectivity to the macroENB with the highest signal strength and the small ENB with the highestsignal strength.
 11. The user equipment of claim 9, wherein, whendynamic association is not allowed, the control unit controls a processof selecting an ENB with the highest signal strength, checking whetheran ENB paired with the selected ENB has a signal strength higher than orequal to a preset threshold, and having, when the paired ENB has asignal strength higher than or equal to the threshold, dual connectivityto the ENB with the highest signal strength and the ENB pairedtherewith, and having, when the paired ENB has a signal strength lowerthan the threshold, single connectivity to the ENB with the highestsignal strength.
 12. The user equipment of claim 9, wherein the controlunit determines the connection configuration for the UE on the basis ofmobility of the UE and necessity of traffic offloading, wherein thecontrol unit makes a connection to a macro ENB first when the UE is ofhigh mobility, and wherein the control unit makes a connection to asmall ENB first when traffic offloading is necessary.
 13. The userequipment of claim 9, wherein the control unit controls receiving cellpair information from a macro ENB or small ENB, wherein the cell pairinformation comprises information on small ENBs pairable with the macroENB and information on macro ENBs pairable with the small ENB, whereinthe control unit controls sending a measurement report to the servingENB on the basis of the determined connection configuration, wherein thecontrol unit controls skipping of sending a measurement report when thecurrent connection configuration is maintained, wherein the control unitcontrols transmitting a measurement report containing measurementinformation for a single frequency band when the connectionconfiguration indicates single connectivity or dual connectivity basedon dynamic association, wherein the control unit controls transmitting ameasurement report containing measurement information for multiplefrequency bands when the connection configuration indicates dualconnectivity based on static association, and wherein the control unitcontrols transmitting a measurement report containing measurementinformation for at least one frequency band associated with a macro ENBor small ENB at the time when the timer for the macro ENB or small ENBexpires.
 14. A base station (ENB) capable of setting a connectionconfiguration for a user equipment (UE) in a wireless communicationsystem supporting dual connectivity, comprising: a transceiver unit tosend and receive a signal to and from the UE; and a control unit tocontrol a process of receiving a measurement report containing resultsof measurement performed by the UE on a macro ENB and a small ENB fromthe UE, determining the connection configuration for the UE on the basisof the measurement report, and sending the determined connectionconfiguration to the UE.
 15. The base station of claim 14, wherein theresults of measurement on a small ENB comprise either results ofmeasurement on all small ENBs having been measured by the UE or resultsof measurement on small ENBs satisfying a preset condition, and whereinthe control unit controls receiving the measurement report at the timewhen the timer for the macro ENB or small ENB expires.